Semiconductor optical source capable of compensating for temperature-induced variation of laser oscillation threshold

ABSTRACT

A semiconductor optical source includes a first laser diode supplied with a signal current pulse and a first bias current for producing an output optical signal pulse in response to the signal current pulse, a first biasing circuit for supplying the first bias current to the first laser diode, a second laser diode supplied with a second bias current for producing an output optical beam in response thereto, a second biasing circuit for supplying the second bias current to the first laser diode; a heat sink for maintaining the first laser diode and the second laser diode at a substantially identical temperature; and a threshold detection circuit for detecting a threshold of laser oscillation of the second laser diode, wherein the threshold detection circuit controls the first biasing circuit such that the first bias current is maintained at a level that is related to the threshold of the second laser diode.

TECHNICAL FIELD

The present invention generally relates to semiconductor opticaldevices, and more particularly to a semiconductor optical source thatuses a laser diode wherein the temperature-induced variation of lasercharacteristics is compensated.

BACKGROUND ART

Laser diodes are used extensively in the human society particularly inthe field of optical telecommunication and optical storage ofinformation. Further, various application of laser diodes are studiedfor example in the field of optical information processing such asoptical computers.

In laser diodes, it is well known that the operational characteristics,particularly the threshold of laser oscillation, change withenvironmental temperature. Thus, in order to compensate for such atemperature-induced variation of the operational characteristics, thelaser diode used for long range telecommunication purposes such as thedevices used for optical submarine cables, use a temperature regulationsystem such that the laser diode is held at a constant temperature.

On the other hand, there exists other applications of laser diodewherein use of such a temperature regulation system is not possible orpreferable. For example, the laser diodes used in the optical wiring ofsupercomputers have to be accommodated in a very limited space and theroom for the temperature regulator is generally not available.Similarly, temperature regulation is not practical for the laser diodesthat are used for the local area network (LAN), laser printers, and thelike, because of the increased cost.

In order to avoid the unwanted temperature-induced fluctuation in theoperation of laser diodes, conventional semiconductor optical sourcesuse a feedback control of optical beam called automatic power control(APC) wherein the bias current is controlled such that the power of theoutput optical pulse of the laser diode is held constant. On the otherhand, such an APC control has a problem in that, associated with thetemperature-induced variation of the threshold of laser oscillation, theextinction ratio of the laser diode is deteriorated with increasingtemperature.

FIG. 1 shows the conventional APC control applied to laser diodes,wherein the horizontal axis represents the drive current while thevertical axis represents the corresponding output optical power.Further, the line designated as P_(L) represents the outputcharacteristics of the laser diode at a low temperature, while the linedesignated as P_(H) represents the output characteristics of the samelaser diode at a high temperature.

Referring to FIG. 1, the drive current is supplied to the laser diode inthe form of a current pulse i_(d), wherein the current pulse i_(d) isbiased to a threshold level i_(thL) that represents the threshold levelof the laser diode at the low temperature, such that the laseroscillation occurs with a minimum, threshold power level when no currentpulse i_(d) is supplied. Further, the magnitude of the current pulsei_(d) is set such that a predetermined output power is achieved inresponse to the drive current pulse i_(d) at the foregoing lowtemperature. In the illustrated example, the magnitude of the currentpulse i_(d) is set to 5 mA.

In the APC control, the biasing of the current pulse i_(d) is subjectedto a feedback control that is achieved in response to the output powerof the laser diode, such that a predetermined output power such as 0.5mW is secured even when the temperature of the laser diode changes. Morespecifically, the bias current added to the current pulse i_(d) ischanged such that the laser diode produces the foregoing predeterminedoutput power in response to the current pulse i_(d).

When the biasing of the drive current pulse i_(d) is set as such, thereoccurs a problem, when the temperature rises, in that the laser diodeproduces the optical output with a substantial power even when thecurrent pulse i_(d) is not supplied. In the illustrated example, thecurrent pulse i_(d) is biased at the level of 20 mA in the illustratedexample, and the laser diode produces the output optical power of about0.25 mW in the absence of the input current pulse i_(d). Thereby, theextinction ratio of the output optical beam is inevitably deteriorated.Associated with the degradation of the extinction ratio, discriminationof the data "0" and the data "1" at the reception side of the opticalcommunication path becomes difficult.

In order to maintain the output power of the optical beam constantirrespective of variation of the temperature, it is also possible tochange the magnitude of the drive current pulse in response to thevariation of temperature. For example, one may fix the level of the biascurrent at i_(thl) and increase the magnitude of the current pulse i_(d)with temperature such that the output power of 0.5 mW is secured evenwhen the temperature increases. According to this approach, one canmaintain a large extinction ratio. However, the foregoing approach has adrawback in that there tends to occur a delay in the timing of theoptical pulse in correspondence to the interval necessary for thecarrier density in the active layer of the laser diode to increase to asufficient level in response to the increase of the drive current fromthe level i_(thl) to the desired level such as 25 mA. In other words,there occurs a time lag in the optical pulse with respect to the timingof the drive current pulse.

DISCLOSURE OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful semiconductor optical source wherein the foregoingproblems are eliminated.

Another and more specific object of the present invention is to providea semiconductor optical source that uses a laser diode wherein thetemperature-induced variation of laser characteristics is compensatedfor while without sacrificing the extinction ratio.

Another object of the present invention is to provide a semiconductoroptical source, comprising: a first laser diode supplied with a signalcurrent pulse and a first bias current for producing an output opticalsignal pulse in response said signal current pulse; first biasing meansfor supplying said first bias current to said first laser diode; asecond laser diode supplied with a second bias current for producing anoutput optical beam in response thereto; second biasing means forsupplying said second bias current to said first laser diode; heat sinkmeans for maintaining said first laser diode and said second laser diodeat a substantially identical temperature; and threshold detection meansfor detecting a threshold of laser oscillation of said second laserdiode, said threshold detection means controlling said first biasingmeans such that said first bias current is maintained at a level that isrelated to said threshold of said second laser diode. According to thepresent invention, one can maintain the bias current of the laser diodeused for optical communication, at a level corresponding to thethreshold level even when the temperature of the laser diode changes.Thereby, the extinction ratio of the optical pulse is held maximumwithout using the temperature regulation device, and the cost as well asthe size of the semiconductor optical source is minimized.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the operational characteristics of aconventional semiconductor optical source;

FIG. 2 is a diagram showing the principle of the present invention;

FIG. 3 is a diagram showing the operation of the semiconductor opticalsource according to a first embodiment of the present invention;

FIG. 4 is a diagram showing the semiconductor optical source accordingto the first embodiment of the present invention;

FIG. 5 is a diagram showing the semiconductor optical source accordingto second and third embodiments of the present invention; and

FIGS. 6(A)-6(C) are block diagrams showing the more detailedconstruction of the semiconductor optical source according to the firstthrough third embodiments of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the principle of the present invention will be described withreference to FIG. 2 showing an essential part of a semiconductor opticalsource that includes a laser diode 1 of which bias current is subjectedto a feedback control process.

Referring to FIG. 2, it will be noted that another laser diode 2, havinga temperature characteristic substantially identical with thetemperature characteristic of the laser diode 1, is provided adjacent tothe laser diode 1 such that the temperature of the laser diode 1 and thetemperature of the laser diode 2 are held substantially at the sametemperature, and a threshold detection circuit 3 is provided fordetecting the threshold level of the laser diode 2. Further, thesemiconductor optical source of FIG. 2 includes a biasing circuit 4 forsupplying a bias current to the laser diode 1, wherein the biasingcircuit 4 is supplied with a control signal indicative of the thresholdbias current of the laser diode 2 from the threshold detection circuit 3and sets the magnitude of the bias current that is supplied to the laserdiode 1 at a level corresponding to the threshold level of the laserdiode 2. Further, an input electrical signal is superposed on the biascurrent thus supplied from the biasing circuit 4 to the laser diode viaa capacitor as illustrated.

According to the present invention, one can set the biasing level of thelaser diode 1 close to the threshold level of the laser diode 1, evenwhen the operational temperature of the semiconductor optical sourcechanges with environmental temperature. Thereby, one can maximize theextinction ratio of the optical signal produced by the laser diode 1.

Next, a first embodiment of the present invention will be described withreference to FIG. 3, wherein shows the operational characteristics of alaser diode of the present embodiment at a low temperature (designatedas P_(L)) and at a high temperature (designated as P_(H)).

Referring to FIG. 3, the vertical axis represents the output opticalpower of the laser diode and the horizontal axis represents the drivecurrent supplied to the laser diode, similarly to the characteristicdiagram of FIG. 3. In FIG. 3, the threshold level of the drive currentat the low temperature is designated by I_(th) (T_(L)), while thethreshold level at the high temperature is designated by I_(th) (T_(H)).

In the present invention, the level of the bias current is controlledclose to the threshold level irrespective of the temperature of thedevice such that the laser diode produces a constant optical power P₀even when the temperature changes. In order to achieve the foregoingcontrol of the drive current in response to the temperature, the presentinvention controls the drive current to the level I_(b) (T_(L)) that isclose to the threshold level when the optical source operates at the lowtemperature, while at the high temperature, the drive current is set atthe level I_(b) (T_(H)) that is also close to the threshold level.Thereby, the output optical power of P₀ that is outputted from the laserdiode in the absence of the input signal pulse, is maintained constantat a level close to zero, irrespective of the temperature.

When a drive current pulse is supplied, on the other hand, the drivecurrent increases by I_(p) that represents the magnitude of the currentpulse. Thus, at the low temperature, the laser diode produces an opticaloutput pulse having a magnitude P₁ (T_(L)) in response to a drivecurrent that is given as a sum of the bias current I_(b) (T_(L)) and thesignal current pulse I_(p) (P₁ (T_(L))=I_(b) +I_(p)), while at the hightemperature, the laser diode produces an optical output pulse having amagnitude P₁ (T_(H)) in response to a drive current that is given as asum of the bias current I_(b) (T_(H)) and the signal current pulse I_(p)(P₁ (T_(H))=I_(b) +I_(p)).

According to the present invention, it will be noted that the outputoptical power P₀ is held close to zero irrespective of the operationaltemperature of the laser diode, and a-large extinction ratio X isguaranteed as compared with the conventional device of which operationalcharacteristic is represented in FIG. 1, wherein the extinction ratio Xis defined as X=1+I_(p) /(I_(b) (T_(H))-I_(th) (T_(H))).

FIG. 4 shows the construction of the semiconductor optical sourceaccording to the first embodiment of the present invention.

Referring to FIG. 4, the device includes a laser diode array 11including a number of laser diodes 11a-11d arranged for producingmultiple-bit optical pulse signals, wherein there is provided another,additional laser diode 12 for detecting the threshold of laseroscillation. There, each laser diode included in the laser diode array11 has a structure identical with each other and is driven by a drivecircuit 14 as usual, while the laser diode 12 has a structure identicalwith the structure of the laser diodes 11a-11d and is carried on acommon substrate 10 that acts also as a heat sink. Thereby, the laserdiode 12 is held at a temperature substantially identical with thetemperature of the laser diodes 11a-11d forming the laser diode array11.

More specifically, the semiconductor optical source of FIG. 4 includes adrive circuit 13 for driving the laser diode 12 and a photodetector 15that is disposed on the same substrate 10 to detect the optical beamproduced by the laser diode 12. In the illustrated example, the drivecircuit 13 controls the laser diode 12 in response to the output voltageof the photodetector 15 such that the output power of the laser diode 12is maintained at the level P₀ that may be set at 0.17 mW, for example.The drive circuit 13 thereby controls a drive circuit 14 of the laserdiode array 11 such that each of the laser diodes 11a-11d is biased atthe bias level identical with the bias level of the laser diode 12.

As illustrated in FIG. 4, the drive circuit 14 supplies the drivecurrent to the laser diodes 11a-11d via inductances L_(a) -L_(d)respectively, and the input signal pulse is added thereto via respectivecapacitors C_(a) -C_(d). Thereby, one can secure a large extinctionratio in the optical pulse signal outputted from the laser diode array11 as explained with reference to FIG. 3. In a typical example where thelaser diode has the threshold current level I_(th) (T_(H)) of 15 mA at25° C. and the drive current pulse I_(p) is given with a magnitude of 30mA, one can secure an extinction ratio of more than 10 dB at 80° C. bysetting the bias current I_(b) (T_(H)) at 18.3 mA.

In the construction of the present embodiment, the optical power P₀ isset at the largest level that is possible so that a reliable APC controlis achieved based upon the power P₀. There, the upper boundary of theoptical power P₀ is determined based upon the constraint that asatisfactory extinction ratio X is obtained at the highest temperaturethat is expected in the operational environment of the semiconductoroptical source.

FIG. 6(A) shows a more detailed diagram of the construction of thesemiconductor optical source according to the first embodiment.

Referring to FIG. 6(A), it will be noted that the drive circuit 13includes an APC unit 13A that is supplied with the output of thephotodetector 15 and a driver unit 13B that drives the laser diode 12under control of the APC unit 13B. More specifically, the APC unit 13Adetects the output level of the photodetector 15 and controls the driverunit 13B such that the bias current supplied to the laser diode 12 isreduced when the output optical power of the laser diode 12 exceeds thepredetermined threshold P₀ that is set close to zero as alreadymentioned. On the other hand, when the output of the laser diode 12 issmaller than the foregoing threshold P₀, the APC unit 13A controls thedriver unit 13B to increase the bias current. Thereby, the APC unit 13Aachieves a feedback control of the optical power of the laser diode 12.

In addition, the APC unit 13A is supplied with a signal indicative ofthe bias current supplied to the laser diode 12 and controls the drivecircuit 14 such that the circuit 14 supplies the bias current to thelaser diodes in the laser diode array 11 with a magnitude that isidentical with the bias current supplied to the laser diode 12. Thereby,the laser diodes 11a-11d forming the laser diode array 11 are all biasedat the level P₀ irrespective of the temperature of the device. It shouldbe noted that the laser diode 12 is carried on the same substrate 10 ofthe laser diode array 11 and the temperature of the device 12 is heldsubstantially identical with the temperature of the laser diode array 11as already mentioned.

Next, a second embodiment of the present invention will be describedwith reference to FIG. 6(B), wherein the semiconductor optical sourcehas a construction similar to the device of FIG. 6(A) and includes thelaser diode array 11 and the laser diode 12 similarly to the firstembodiment. In the present embodiment, it will be noted that the outputof the photodetector 15 is supplied to a threshold-level detection unit13A' for detection of the threshold level of laser oscillation of thelaser diode 12 while changing the bias current to the laser diode 12 upand down by a bias swing unit 13B', wherein the bias swing unit 13B'increases and decreased the bias current repeatedly in a range extendingfrom zero to a predetermined upper limit that is selected sufficientlylarger than the threshold level I_(b) (T_(H)).

More specifically, the detection unit 13A' monitors the optical outputof the laser diode 12 while the drive current supplied thereto ischanged up and down and detects a sudden change in the output opticalpower of the laser diode 12. For example, the output optical power ofthe laser diode 12 increases suddenly when the bias current exceeds thethreshold level in correspondence to the commencement of laseroscillation. Further, the unit 13A' produces the trigger signal when thebias current has decreased below the threshold level. In response to thedetection of such a sudden change in the optical output of the laserdiode 12, the unit 13A' controls the drive circuit 14 such that thelaser diodes in the laser diode array 11 are biased at the levelidentical with the threshold level of the laser diode 2. In other words,the laser diodes 11a-11d in the laser diode array 11 are all biased tothe threshold level of laser oscillation even when the temperature haschanged. It should be noted that the unit 13A' of FIG. 6(B) is suppliedwith a signal indicative of the magnitude of the bias current that issupplied to the laser diode 12, from the bias swing unit 13B', andlatches the same in response to the detection of the foregoing suddenchange of the optical output of the laser diode 12. The detected biascurrent is updated each time the sudden change of the optical output isdetected.

Next, a third embodiment of the present invention will be described withreference to FIG. 5.

Referring to FIG. 5, it will be noted that the photodetector 15 used inthe first and second embodiments is eliminated and a bias circuit 23 isused for biasing the laser diode 12 in place of the unit 13. There, thebias circuit 23 detects the drive current supplied to the laser diode 12together with the drive voltage and detects the sudden change of thedrive current with respect to the voltage. Further, the circuit 23latches the bias current corresponding to such a sudden change andsupplies a control signal indicative of the magnitude of the biascurrent to the drive circuit 14. The drive circuit 14 in turn controlsthe bias current supplied to the laser diodes 11a-11d in the laser diodearray 11 at the level indicated by the control signal provided by thebias circuit 23, and the laser diodes in the array 11 are all biasedclose to the threshold level of laser oscillation. Similarly to thesecond embodiment, the latched value representing the magnitude of thebias current is updated each time the sudden change of the drive currentis detected.

FIG. 6(C) shows the block diagram of the laser diode of the presentembodiment.

Referring to FIG. 6(C), the bias circuit 23 includes a voltage detectionunit 23C for detecting the voltage that is applied to the laser diode12, and a bias swing circuit 23B similar to the circuit 13B' changes thebias current up and down repeatedly. Further, a threshold detection unit23A is supplied with a signal indicative of the magnitude of thedetected drive current and voltage respectively from the units 23B and23C and detects the sudden change in the drive current. The unit 23Afurther latches the magnitude of the drive current and supplies acontrol signal indicative of the latched drive current to the biascircuit 14. The bias circuit 14 thereby supplies the bias current to thelaser diodes forming the laser diode array 11.

According to the present embodiment, the threshold is detected with highprecision based upon the differential voltage versus currentcharacteristic curve of the laser diode, and one can maintain the biaslevel of the laser diodes closer to the threshold of laser oscillation.

In the foregoing embodiments, it is assumed that the laser diodes11a-11d forming the array 11 and the laser diode 12 are held at the sametemperature. However, this requirement is not mandatory in the presentinvention and the temperature of the laser diode 12 may be differentfrom the temperature of the laser diode array 11, provided that bothtemperatures are related with each other according to a knownrelationship. Further, the structure of the laser diode 12 is notnecessarily be exactly identical with the laser diodes forming the array11. Furthermore, one may use one of the laser diodes forming the laserdiode array 11 for the laser diode 12.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

Industrial Applicability

According to the present invention, one can maintain a satisfactoryextinction ratio in the output optical signal of laser diodes even whentemperature regulation of the laser diodes is omitted. Thus, thesemiconductor optical source of the present invention is usefulparticularly in the applications such as optical wiring ofsupercomputers or local area network wherein the room for thetemperature regulation device is not available or the increase of costassociated with the temperature regulation is not desirable.

I claim:
 1. A semiconductor optical source, comprising:a first laserdiode supplied with a signal current pulse and a first bias current forproducing an output optical signal pulse in response to said signalcurrent pulse; first biasing means for supplying said first bias currentto said first laser diode; a second laser diode supplied with a secondbias current for producing an output optical beam in response thereto;second biasing means for supplying said second bias current to saidsecond laser diode; heat sink means for maintaining said first laserdiode and said second laser diode at a substantially identicaltemperature; and control means for controlling said first biasing meanssuch that said first bias current is maintained at a level that isrelated to a threshold of said second laser diode wherein said controlmeans controls said second biasing means in response to an optical powerof said optical beam produced by said second laser diode by causing saidsecond biasing means to supply said second bias current to said secondlaser diode such that said optical power of said optical beam ismaintained at a predetermined level, said control means furthercontrolling said first biasing means such that said first biasing meanssupplies said first bias current with a level substantially identicalwith said second bias current to said first laser diode, saidpredetermined level of said optical power being determined such that anextinction ratio of said optical signal .pulse that is produced by saidfirst laser diode exceeds a desired value at a highest expectedoperational environment temperature of said semiconductor opticalsource.
 2. A semiconductor optical source as claimed in claim 1, whereinsaid control means includes a photodetector disposed on said heat sinkmeans for receiving said optical beam produced by said second laserdiode.
 3. A semiconductor optical source as claimed in claim 1, whereinsaid semiconductor optical source includes a plurality of laser diodesprovided on said heat sink means and forming an array, said first laserdiode and said second laser diode being included in said array.
 4. Asemiconductor optical source as claimed in claim 3, wherein saidsemiconductor optical source includes a plurality of laser diodesprovided in said heat sink means in a form of an array as said firstlaser diode.
 5. A method for controlling a semiconductor optical source,comprising the steps of:maintaining a first laser diode and a secondlaser diode at a substantially identical temperature; maintaining asecond bias current supplied to said second laser diode at apredetermined level; maintaining a first bias current supplied to saidfirst laser diode at a level substantially identical with saidpredetermined level of said second bias current; and supplying a currentpulse signal to said first laser diode in addition to said first biascurrent to produce an optical pulse signal in correspondence to saidcurrent pulse signal wherein said predetermined level of said secondbias current being set such that said second laser diode produces anoutput optical beam with a predetermined magnitude set such that saidfirst laser diode produces said optical pulse signal with an extinctionratio that exceeds a desired value at a highest expected environmenttemperature in which the semiconductor optical source is operated.
 6. Amethod as claimed in claim 5, wherein said predetermined level of saidsecond bias current is set in correspondence to a sudden increase ofoptical power of an optical beam produced by said second laser diode.